Exchange 2003 to 2007: Mailbox and User

1) There are Exchange 2003 in 2 domains; Boba and DingDong Domain
2) I want to migrate to Exchange 2007
3) There are suggestion (from the Experts from EE) that i have to move the Mailbox and also move the user
4) I am confused (related to this); per my understanding the Users are at Active Directory and Nothing to do with the Microsoft Exchange
5) Per my experience of moving exchange 2003 to exchange 2007 (only one domain); i do NOT need to move the User
6) Would experts explain why i have to move the users (if it is more than one domain)?
7) Thank you
tjie

The Mailbox server role can coexist with Exchange Server 2003 and Microsoft Exchange 2000 Server mailbox servers. For Exchange 2007 and Exchange 2000 Server or Exchange Server 2003 mailbox servers to coexist, you must be able to send mail among the mailboxes. Exchange 2007 uses the Hub Transport server to send mail. A Hub Transport server must be deployed in each Active Directory site that contains an Exchange 2007 Mailbox server.

You can move mailboxes among Exchange 2000 Server, Exchange Server 2003, and Exchange 2007. For information about how to move mailboxes, see Moving Mailboxes.

If you move a mailbox from Exchange 2000 Server or Exchange Server 2003 to Exchange 2007, and the mailbox is part of an e-mail address policy, the e-mail addresses for that mailbox is automatically updated based on the configuration of the e-mail address policy. If the mailbox had a primary SMTP address that differs from the e-mail address that is enforced by the e-mail address policy, that SMTP address becomes a secondary SMTP address and the e-mail address generated by the e-mail address policy becomes the primary SMTP address.

This behavior differs from Exchange 2003 and Exchange 2000. In Exchange 2003 and Exchange 2000 , the e-mail address policy is not applied to a mailbox when it is moved. To prevent accidentally changing the primary SMTP address of a mailbox in an Exchange 2007 environment, you must configure the mailbox so that is does not automatically update e-mail addresses based on e-mail address policy.

You can replicate public folder data between Exchange 2007 and Exchange 2000 Server or Exchange Server 2003 public folder databases. To do this, you must create a replica of the public folder using either the Exchange 2000 Server or Exchange Server 2003 versions of Exchange System Manager. The routing group connector between Exchange 2007 and Exchange 2003 can be configured to enable public folder referrals.

You aren't getting a good answer because you haven't created a very clear question.

Exchange is extremely reliant on Active Directory. You can't have mailboxes without users. If you have set up a new domain and installed Exchange into it, you don't have to have the users in it, but you'd have to have a trust relationship between the new AD domain and the old user AD domains. If there is a trust relationship, you can assign users in your existing domains to resources in your "resource domain" that is hosting Exchange.

So when you refer to a "domain" are you talking about DNS domains or AD domains? If they are DNS domains, then your answer will be different.

In general, you will want to keep things as simple as possible for the future even if that means doing a little more work in the short term, which means you will have a flat (single) domain structure as your goal if possible.

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